1. Field of the Invention
The present invention relates to a liquid-crystal display device which is excellent in light utilizing efficiency and which provides a brilliant display easy to see.
The present application is based on Japanese Patent Application No. Hei. 11-143495, which is incorporated herein by reference.
2. Description of the Related Art
There is investigation of a liquid-crystal display device which is used both in a reflection and a transmission mode so that the device can be visually recognized in use of the transmission type liquid-crystal display device in a dark place, or the like, by addition of an illuminator while the merit of a reflection type liquid-crystal display device of small power consumption is used widely. For example, a liquid-crystal display device using a semi-transmission type reflection plate, a liquid-crystal display device in which a back light used in a transmission type liquid-crystal display device is arranged as a front light on the visual recognition side of liquid-crystal cells, and so on, have been proposed.
The system utilizing such a semi-transmission type reflection plate, however, had a disadvantage that the system in either mode was inferior in brightness to the reflection or transmission type device for exclusive use because light was separated into reflected and transmitted light by a half-mirror effect. In consideration of the disadvantage, there was a proposal to make an improvement by use of a reflection polarizer for reflecting polarized light selectively so that the sum of reflectivity and transmissivity was able to exceed 100%. However, there were problems in that the efficiency of light utilizing efficiency in a transmission mode was reduced to be not higher than 50% because of absorption by a light absorber disposed for preventing both display inversion between reflection and transmission and emphasis of a black display, and display was hard to see in the evening twilight in either mode.
On the other hand, there was a problem in that the front light system was apt to make a display darker in a transmission mode than the general transmission type liquid-crystal display device because light went to light-crystal cells, or the like, and came back. Accordingly, an injury of a light pipe or a contaminant thereon became conspicuous as a bright spot, and display contrast was lowered by leakage of light from an upper surface of the light pipe.
An object of the present invention is to develop a liquid-crystal display device of good visual recognition which is excellent in brightness and uniformity of brightness both in reflection and transmission modes, and in which display inversion, lowering of contrast due to leakage of light, and the visual sense of dazzling light do not occur.
According to the present invention, there is provided a liquid-crystal display device comprising: a light pipe provided with light output means formed on an upper surface of the light pipe; a light source disposed near to an incident side surface of the light pipe so that light incident on the light pipe from the light source is delivered from a lower surface of the light pipe through the light output means; a reflection layer disposed on the lower surface of the light pipe so that reflected light of the output light is transmitted through the upper surface; a light diffusion layer having polarization-keeping characteristic; and a liquid-crystal shutter disposed on the upper surface of the light pipe through the light diffusion layer, the liquid-crystal shutter including liquid-crystal cells, and at least one polarizing plate.
According to the present invention, there is provided a structure in which the light pipe and the polarization-keeping light diffusion layer are disposed between the reflection layer and the liquid-crystal cells. Lowering of light utilizing efficiency in a reflection mode is no more than absorption loss, reflection loss, etc. due to the light pipe and the light diffusion layer. Lowering of the degree of polarization due to the light diffusion layer is also slight. As a result, brilliance approximately equivalent to that of a background-art reflection type liquid-crystal display device can be achieved in a reflection mode. Moreover, brilliance not at all inferior to that of a background-art transmission type liquid-crystal display device can be achieved in a transmission mode. In addition, inversion of display does not occur between reflection and transmission.
Moreover, lowering of contrast caused by leakage light from the light pipe does not occur. By the effect of diffusion through the light diffusion layer, the display is made excellent in uniformity of brightness. At the same time, the visual sense of dazzling light caused by both bright-line-shaped light emission and metallic reflection is suppressed. Moreover, an in-display image of the light output means formed on the light pipe is weakened, so that the pattern of the light output means is prevented from becoming visible. As a result, a liquid-crystal display device of a good visual recognition property can be obtained.
Moreover, a light path in the light pipe in the transmission mode can be elongated by provision of the light output means on the upper surface of the light pipe. Hence, the spread of light is widened, so that the intensity of a bright line can be relaxed. This effectively acts on both prevention of occurrence of moire and enhancement of uniformity of brilliance advantageously, so that the reflection layer can be disposed on the lower surface of the light pipe so as to be brought into contact with and integrated with the light pipe through a tacky layer, or the like, easily. If such light output means is provided on the lower surface of the light pipe, it is necessary to provide an independent reflection plate separately in terms of the maintenance of the function of the light output means. As a result, the structure of the device is complicated because of increase in the number of parts and arrangement and fixation of the reflection plate. As a result, there is a disadvantage that the weight of the device becomes heavy because a thick support is required for preventing disorder of display from being caused by generation of wrinkles.
Further, in the case of a light pipe having light output means constituted by slopes such as prism-like irregularities, the light pipe is excellent in directivity of reflected light through the slopes. Accordingly, light advantageous to visual recognition in a transmission mode can be formed efficiently, so that more brilliant display can be obtained. The light pipe is further excellent both in incident efficiency of external light and in transmission efficiency after reflection thereof. Accordingly, more brilliant display can be obtained also in a reflection mode by light emission excellent both in light utilizing efficiency and in uniformity. Further, generation of moire due to the foregoing directivity can be suppressed by oblique arrangement of the light output means, so that inhibition of visual recognition due to dazzling light can be prevented from being generated.
As described above, in the case of a light pipe provided with scatter type light output means such as dots, embossed irregularities, or the like, output light is emitted at a large angle of about 60 degrees, so that display in a transmission mode is dark and hard to see in a frontal (vertical) direction. If a prism sheet is disposed for the purpose of controlling the light path, light is scattered in a reflection mode, so that a display is made very dark because a large part of light does not contribute to visual recognition. If a diffusion layer of a strong diffusing property is disposed in order to prevent dots, or the like, from being visible too clearly, a dark display is obtained in a reflection mode because both incident and reflected light thereof from the reflection layer are also scattered in the reflection mode.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.